Today, optical transmission system rely widely on wavelength division multiplexing, where individual wavelength channels are arranged in a regular wavelength grid of a certain wavelength spacing of typically 50 or 100 GHz. Typically, DPSK (differential phase-shift keying) or DQPSK (differential quadrature phase-shift keying) would be used to modulate a carrier on each wavelength channel. Not all of the waveband can be utilized for signal transmission, since in order to avoid signal overlap and crosstalk, a wavelength channel is separated in the frequency domain from a neighboring wavelength channel through a guard interval, which is in the range of 30% of the channel spacing.
Recently, a new modulation format known as orthogonal frequency division multiplexing (OFDM) has gained increased interest as candidate for future high-speed and high capacity optical transmission due to its high spectral efficiency and its resilience in the presence of fiber dispersion and polarization mode dispersion (PMD).
For OFDM transmission, a WDM wavelength channel is subdivided into equidistant sub-channels, which all together carry the information content of a data signal in a parallel fashion at a lower symbol rate.
An OFDM signal is typically generated by means of an inverse fast-Fourier transform: Parallel data streams, each data stream corresponding to one sub-channel, are mapped to parallel symbol streams using a specific modulation scheme (e.g., phase-shift keying—PSK, or quadrature amplitude modulation—QAM), and then fed to an IFFT-unit for performing an inverse fast Fourier-transform (IFFT). In the receiver, the process is reversed by feeding an FFT-unit performing a fast Fourier-transform (FFT).